Integrated circuits may be advantageously utilized in modern automotive electrical systems, for example, in ignition systems or in seat belt interlock systems, affording substantial cost savings. However, the automotive environment has been found to be an exceptionally harsh one for semiconductor circuits in general, and for integrated circuits in particular. As a result, unexpected problems and requirements have arisen in the design of integrated circuits which must perform reliably in automotive electrical systems, and in other high-noise environments. A wide range of temperatures may occur in the automotive environment. Further, a wide range of spurious signals typically occur throughout the wiring of an automotive electrical system. For example, relatively low energy spurious signals of either positive or negative polarity having magnitudes of several hundred volts, hereinafter referred to as "noise" signals, typically occur on wiring lines connecting various sensors to input terminals of integrated circuit devices. Such noise signals may cause malfunctions in the operation of prior art integrated circuit devices, or may even cause destruction of them, and further may destroy discrete semiconductor devices such as power transistors controlled by the integrated circuit. Further, discontinuities in the main power lines of an automotive electrical system, such as interruptions in the connection to the 12 volt automobile battery, may cause severe, high-energy transient voltages, hereinafter called "load dump" voltages, of over 100 volts to occur on the main power lines. The load dump transient voltages may destroy the integrated circuit devices of the prior art in the absence of expensive external protective measures.
Interface circuits of the prior art have had two major shortcomings for utilization in high noise environments. A shortcoming of some prior art interface circuits is that an N-type epitaxial region is directly connected to an input or output terminal of an integrated circuit chip, which input or output terminal may be connected to a long wire which passes through a high noise environment, and thereby has large magnitude noise pulses thereon. If a large negative noise signal occurs on the terminal, the PN junction between the N-type epitaxial region and the P-type substrate is forward biased, and minority carriers (electrons) are injected into the substrate, and in some cases are collected by other nearby reverse biased N-type epitaxial regions. This frequently causes malfunction of the circuit, especially if flip-flop devices or memory devices are on the chip. Another problem typically occuring with prior art interface circuits, including those used in automotive circuits, is that under some conditions a very high impedence appears at the terminal, and a low energy noise signal may develop sufficiently high voltage to cause the circuit to respond to the noise signal as if it were an information signal. This situation is especially likely to occur when external terminals of two integrated circuits are connected together to a long wire in a high noise environment so that an output stage on one chip drives both the input stage to another chip and the long wire. The output transistor of the first chip may be off at some time during the circuit operation causing a high impedence between the long wire and ground. Noise signals may then have sufficient magnitude to cause malfunctions in circuit operation. The present invention solves these two major shortcomings of the prior art interface circuits by providing interface circuitry which prevents injection of minority carriers into the substrate and always presents a low impedence at external terminals during operating conditions.